arXiv subjects
Publications and source records attributed to Xiang Liu.
Stimulated by recent observations of the excited bottom-strange mesons $B_{s1}$ and $B_{s2}^*$, we calculate the semileptonic decays $B_{s0}, B_{s1}^{\prime}, B_{s1}, B_{s2}^*\to [D_s(1968), D_{s}^*(2112), D_{sJ}(2317), D_{sJ}(2460)]\ell\barν$, which is relevant for the exploration of the potential of searching these semileptonic decays in experiment.
It is shown that the Lambda(c+)-Lambda(c-) signal recently reported by the Belle Collaboration (ARXIV:0807.4458) contains clear signs of the psi(5S) and the psi(4D) c-cbar vector states, and also some indication for the masses and widths of the psi(6S) and psi(5D). Moreover, it is argued that the threshold behaviour of the Lambda(c+)-antiLambda(c-) cross sections suggests the presence of the hitherto undetected psi(3D) state not far below the Lambda(c+)-Lambda(c-) threshold.
We study the production of $Z^\pm(4430)$ at a nucleon-antinucleon scattering experiment. Considering the PANDA experiment to be an ideal platform to explore the production of the charmonium and charmonim-like states, we suggest the forthcoming PANDA experiment to pay attention to the production of $Z^\pm(4430)$.
In this review, we discuss some interesting issues in charm physics which is full with puzzles and challenges. So far in the field there exist many problems which have not obtained satisfactory answers yet and more unexpected phenomena have been observed at the present facilities of high energy physics. Charm physics may become an ideal place for searching new resonances and studying non-perturbative QCD effects, moreover probably is an area to explore new physics beyond the Standard Model. More data will be available at BESIII, B-factories, LHC and even future ILC which may open a wide window to a better understanding of the nature.
We perform a systematic study of the possible molecular states composed of a pair of heavy mesons such as $D\bar D$, $D^\ast\bar D$, $D^\ast \bar D^\ast$ in the framework of the meson exchange model. The exchanged mesons include the pseudoscalar, scalar and vector mesons. Through our investigation, we find that (1) the structure X(3764) is not a molecular state; (2) There exists strong attraction in the range $r < 1$ fm for the $D^*\bar D^*$ system with $J=0, 1$. If future experiments confirm $Z^+(4051)$ as a loosely bound molecular state, its quantum number is probably $J^{P}=0^+$. Its partner state $Φ^{**0}$ may be searched for in the $π^0χ_{c1}$ channel; (3) The vector meson exchange provides strong attraction in the $D^\ast \bar D$ channel together with the pion exchange. A bound state solution exists with a reasonable cutoff parameter $Λ\sim 1.4$ GeV. X(3872) may be accommodated as a molecular state dynamically although drawing a very definite conclusion needs further investigation; (4) The $B^\ast \bar B$ molecular state exists.
We reexamine whether $Z^+(4430)$ could be a $D_1'-{D}^*$ or $D_1-{D}^*$ molecular state after considering both the pion and $σ$ meson exchange potentials and introducing the form factor to take into account the structure effect of the interaction vertex. Our numerical analysis with Matlab package MATSLISE indicates the contribution from the sigma meson exchange is small for the $D_1'-{D}^*$ system and significant for the $D_1-{D}^*$ system. The S-wave $D_1-\bar{D}^*$ molecular state with only $J^{P}=0^-$ and $D_1'-{D}^*$ molecular states with $J^P=0^-,1^-,2^-$ may exist with reasonable parameters. One should investigate whether the broad width of $D_1'$ disfavors the possible formation of molecular states in the future. The bottom analog $Z_B$ of $Z^+(4430)$ has a larger binding energy, which may be searched at Tevatron and LHC. Experimental measurement of the quantum number of $Z^+(4430)$ may help uncover its underlying structure.
We study the mass of the state Y(2175) of J^{PC} = 1^{--} in the QCD sum rule. We construct both the diquark-antidiquark currents (ss)(s_bar s_bar) and the meson-meson currents (s_bar s)(s_bar s). We find that there are two independent currents for both cases, and derive the relations between them. The OPE convergence of these two currents is sufficiently fast, which enables us to perform good sum rule analysis. Both the SVZ sum rule and the finite energy sum rule lead to a mass around 2.3+-0.4 GeV, which is consistent with the observed mass within the uncertainties of the present QCD sum rule. The coupling of the four-quark currents to lower lying states such as ϕ(1020) turns out to be rather small. We also discuss possible decay properties of Y(2175) if it is a tetraquark state.
This paper has been withdrawn by the authors as a consequence of their misinterpretation of the BES experiment in 0807.0494
To testify the validity of the perturbative QCD (pQCD) and investigate its application range, one should look for a suitable process to do the job. $B\to J/ψD$ is a promising candidate. The linear momentum of the products is relatively small, so that there may exist a region where exchanged gluons are soft and the perturbative treatment may fail, so that the non-perturbative effect would be significant. We attribute such non-perturbative QCD effects into the long-distance final state interaction (FSI) which is estimated in this work. We find that the contribution from the FSI to the branching ratio is indeed sizable and may span a rather wide range of $10^{-6}\sim 10^{-5}$, and cover a region where the pQCD prediction has the same order. A more accurate measurement on its branching ratio may provide important information about the application region of pQCD and help to clarify the picture of the inelastic rescattering (i.e. FSI) which is generally believed to play an important role in B decays.
After taking into account both the pion and sigma meson exchange potential, we have performed a dynamical calculation of the $D^0\bar{D}^{\ast0}$ system. The $σ$ meson exchange potential is repulsive from heavy quark symmetry and numerically important for a loosely bound system. Our analysis disfavors the interpretation of X(3872) as a loosely bound molecular state if we use the experimental $D^\ast Dπ$ coupling constant $g=0.59$ and a reasonable cutoff around 1 GeV, which is the typical hadronic scale. Bound state solutions with negative eigenvalues for the $D\bar{D}^\ast$ system exist only with either a very large coupling constant (two times of the experimental value) or a large cutoff ($Λ\sim 6$ GeV or $β\sim 6$ GeV$^2$). In contrast, there probably exists a loosely bound S-wave $B\bar{B}^\ast$ molecular state. Once produced, such a molecular state would be rather stable since its dominant decay mode is the radiative decay through $B^\ast\to B γ$. Experimental search of these states will be very interesting.
The two-body open charm decays $Z^+(4430)\to D^{+}\bar{D}^{*0}, D^{*+}\bar{D}^{0}, D^{*+}\bar{D}^{*0}$ occur through the re-scattering mechanism and their branching ratios are strongly suppressed if $Z^+(4430)$ is a $D_1\bar D^*$ molecular state. In contrast, $Z^+(4430)$ falls apart into these modes easily with large phase space and they become the main decay modes if $Z^+(4430)$ is a tetraquark state. Experimental search of these two-body open charm modes and the hidden charm mode $χ_{cJ}ρ$ will help distinguish different theoretical schemes.
It is well known that the final state interaction plays an important role in the decays of $B$-meson. The contribution of the final state interaction which is supposed to be long-distance effects, to the concerned processes can interfere with that of the short-distance effects produced via the tree and/or loop diagrams at quark-gluon level. The interference may provide a source for the direct CP violation $\mathcal{A}_{CP}$ in the process $B_{c}^{+}\to D^{0}π^{+}$. We find that a typical value of $\mathcal{A}_{CP}$ when the final state interaction effect is taken into account can be about -22% which is different from that without the final state interaction effect. Therefore, when we extract information on CP violation from the data which will be available at LHCb and the new experiments in $B$-factories, the contribution from the final state interaction must be included. This study may be crucial for searching new physics in the future.
In this talk, we review our recent work about the dynamical studies of $Z^+(4430)$ and X(3872). $Z^+(4430)$ can not be explained as a $D_1'D^*$ or $D_1D^*$ molecular state only considering one pion exchange potential without the cutoff, which needs to be confirmed by introducing sigma exchange potential and adding the cutoff in the effective potential. One also excludes the possibility of X(3872) as a $DD^*$ molecular state by one pion and one sigma exchanges with the cutoff. Fortunately there exists an S-wave $BB^*$ bound state with $J^{PC}=1^{++}$. we suggest future experiment to search this state.
A Monte Carlo framework, MaGe, has been developed based on the Geant4 simulation toolkit. Its purpose is to simulate physics processes in low-energy and low-background radiation detectors, specifically for the Majorana and Gerda $^{76}$Ge neutrinoless double-beta decay experiments. This jointly-developed tool is also used to verify the simulation of physics processes relevant to other low-background experiments in Geant4. The MaGe framework contains simulations of prototype experiments and test stands, and is easily extended to incorporate new geometries and configurations while still using the same verified physics processes, tunings, and code framework. This reduces duplication of efforts and improves the robustness of and confidence in the simulation output.
It is interesting to search for new physics beyond the standard model at LHCb. We suggest that weak decays of doubly charmed baryon such as $Ξ_{cc}(3520)^+, Ξ_{cc}^{++}$ to charmless final states would be a possible signal for new physics. In this work, we consider two models, i.e. the unparticle and $Z'$ as examples to study such possibilities. We also discuss the cases for $Ξ^0_{bb}, Ξ_{bb}^-$ which have not been observed yet, but one can expect to find them when LHCb begins running. Our numerical results show that these two models cannot result in sufficiently large decay widths, therefore if such modes are observed at LHCb, there must be a new physics other than the unparticle or $Z'$ models.
Considering two molecular state assumptions, i.e. S-wave $\barΛ-K^-$ and S-wave $\barΣ^0-K^-$ molecular states, we study the possible decays of $\bar N_X(1625)$ that include $\bar N_X(1625)\to K^{-}\barΛ, π^{0}\bar{p}, η\bar{p}, π^{-}\bar{n}$. Our results indicate: (1) if $\bar N_{X}(1625)$ is $\barΛ-K^-$ molecular state, $K^{-}\barΛ$ is the main decay modes of $\bar N_{X}(1625)$, and the branching ratios of the rest decay modes are tiny; (2) if $\bar N_{X}(1625)$ is $\barΣ^0-K^-$ molecular state, the branching ratio of $\bar N_{X}(1625)\to K^{-}\barΛ$ is one or two order smaller than that of $\bar N_{X}(1625)\to π^{0}\bar{p}, η\bar{p}, π^{-}\bar{n}$. Thus the search for $\bar N_X(1625)\to π^{0}\bar{p}, η\bar{p}, π^{-}\bar{n}$ will be helpful to shed light on the nature of $\bar N_X(1625)$.
Since $Z^+(4430)$ lies very close to the threshold of $D^\ast{\bar D}_1$, we investigate whether $Z^+(4430)$ could be a loosely bound S-wave state of $D^\ast{\bar D}_1$ or $D^\ast{\bar D}^\prime_1$ with $J^P=0^-, 1^-, 2^-$, i.e., a molecular state arising from the one-pion-exchange potential. The potential from the crossed diagram is much larger than that from the diagonal scattering diagram. With various trial wave functions, we notice that the attraction from the one pion exchange potential alone is not strong enough to form a bound state with realistic pionic coupling constants deduced from the decay widths of $D_1$ and $D^\prime_1$.
Very recently a new enhancement around 4.05 GeV was observed by Belle experiment. In this short note, we discuss some possible assignments for this enhancement, i.e. $ψ(3S)$ and $D^*\bar{D}^*$ molecular state. In these two assignments, Y(4008) can decay into $J/ψπ^0π^0$ with comparable branching ratio with that of $Y(4008)\to J/ψπ^+π^-$. Thus one suggests high energy experimentalists to look for Y(4008) in $J/ψπ^0π^0$ channel. Furthermore one proposes further experiments to search missing channel $D\bar{D}$, $D\bar{D}^*+h.c.$ and especially $χ_{cJ}π^+π^-π^0$ and $η_cπ^+π^-π^0$, which will be helpful to distinguish $ψ(3S)$ and $D^*\bar{D}^*$ molecular state assignments for this new enhancement.